The lake lies at an altitude of 420 m, is 105 km long and 22–48 km wide, with a maximum depth of 15 m. Its major tributaries are the Kara Irtysh (Black Irtysh) and Kendyrlyk from the east; its only outlet is the Irtysh River (or White Irtysh). The lake is generally frozen from the beginning of November to the end of April, but still has an abundance of fish. Since the construction of the Bukhtarma dam on the Irtysh downstream from the Zaysan, the lake has risen 6 m (20 ft) above its natural level. As the result, the area of lake essentially increased (nearly doubled: from about 1,800 km² to 3,500 km²,[1] or even to 5,000 km²),[2] thus, in some sources the lake is indicated as the "part" of artificial reservoir.[3]

The lake has existed since the late Cretaceous Period.[citation needed] Commonly, Lake Baikal is considered to be the oldest lake on the planet (about 25 million years).[4] The approximate duration of Cretaceous Period is from 136 to 65 million years before present,[5] and, hence, if the Lake Zaysan existed from "the late Cretaceous Period", that means Zaysan must be older than 65 million years, and is the oldest lake on the planet, nearly three times older than Baikal. The direct indication of the Lake Zaysan's age is difficult to find, although some geological studies of the Zaysan Basin have been reviewed.[6] Artificial reservoirs cover large surrounding areas. Modern geological analysis of the entire field, apparently, supports an exceptionally old age for Lake Zaysan.[7][8]

The first Russian to reach the area was Ivan Bukholts who ascended the Irtysh to build a fort and search for gold. In 1715 he was driven back downriver by the Oirats, who has established the Zunghar Khanate in the region.

The Chinese Qing Empireconquered the Zunghar state in the 1750s. This prompted an increase in the Russian authorities' attention to their borderland; in 1756, the Orenburg Governor Ivan Neplyuyev even proposed the annexation of the Lake Zaysan region, but this project was forestalled by Chinese successes.[9] Concerns were raised in Russia (1759) about the (theoretical) possibility of a Chinese fleet sailing from Lake Zaysan down the Irtysh and into Western Siberia. A Russian expedition visited Lake Zaysan in 1764, and concluded that such a riverine invasion would not be likely. Nonetheless, a chain of Russian pickets was established on the Bukhtarma River, north of Lake Zaysan.[10] Thus the border between the two empires in the Irtysh basin became roughly delineated, with a (sparse) chain of guard posts on both sides.

The situation on the Zaysan in the mid-19th century is described in a report by A.Abramof (1865). Even though the Zaysan region was recognized by both parties as part of the Qing Empire, it had been annually used by fishing expeditions sent by the Siberian Cossack Host. These summer expeditions started in 1803, and in 1822-25 their range was expanded through the entire Lake Zaysan and to the mouth of the Black Irtysh. Through the mid-19th century, the Qing presence on the upper Irtysh was mostly limited to the annual visit of the Qing amban from Chuguchak to one of the Cossacks' fishing stations (Batavski Piket).[11]

The border between the Russian and the Qing empires in the Irtysh basin was established along the line fairly similar to China's modern border with Russia and Kazakhstan by the Convention of Peking of 1860.[12] The actual border line pursuant to the convention was drawn by the Protocol of Chuguchak (1864), leaving Lake Zaysan on the Russian side.[13][14] The Qing Empire's military presence in the Irtysh basin crumbled during the Dungan revolt (1862–77). After the fall of the rebellion and the reconquest of Xinjiang by Zuo Zongtang, the border between the Russian and the Qing empires in the Irtysh basin was further slightly readjusted, in Russia's favor, by the Treaty of Saint Petersburg (1881).